Your parties Wizard isn't going to be happy when the Balor beheads him for 74+3d12 damage. Taking off 27 damage off that is a hollow victory for you at best - the Wizard is dead when the rest of the monsters just beat him to death.
that's a poor example. a very specific monster and specifically a critical hit condition. obviously the Battlemind is going to be much better in that situation, mostly because the Balor is huge and staying adjacent for Mind Spiking should be really easy [assuming everyone has fire resistance].
but the shielding Swordmage doesn't suck against it as much as your example describes. the SSM can hit it every turn with frigid blade, reduce its land movement to a maximum four square charge. try to keep it from even getting within range of the wizard, who should also be pushing it back and trying to stun it or what have you - then use the damage reduction on smaller creatures that might get through.
plus you really should be dropping Swordmage Shielding Fire on that thing. if you can't slow it down enough, like when you inevitably miss with Frigid Blade, you Prevent 27 of its damage and deal 27 to it. on a non-crit the Balor averages 41 damage, so it'll deal around 14 and take 27. that's not a bad deal.
To use a non-brute and non-Balor example, there are skirmishers who are happily running around doing a close burst 1, 4d10+16 fire and force +15 ongoing damage twice on their turn. These powers will shred most of the party and there are 4 other monsters that have similar capabilities.
this is the real problem with the SSM. can't prevent ongoing damage riders.
you party is going to need to have ways of dealing with this at Epic. leaders that can hand out pre-emptive save attempts, transference weapons and other items that deal with conditions. SSMs can draw out the battle and that enhances the potency of ongoing damage. You need to be aware of this specific weakness so you can cover for it.
it is also likely where any second defender in your party is probably going to want to take the lead by marking the ongoing damage dealers while you mark everything else. it's a team game.
assuming they aren't just killing you (which monsters are more than capable of doing to any defender now)
you've got to know when to parry gauntlets+total defense or turn invisible or use temp hit points/damage negation powers or get healed or spark your epic level regeneration or whatever of the many tricks you have selected for dealing with getting mauled but that's just part of being a defender now. you need the ability to turtle.